Secretary-General Antonio Guterres says he hopes the battle for Hudaida, where Yemen's main port lies, can be avoided.

The United Nations is engaged in "intense" negotiations with the United Arab Emirates and Saudi Arabia in a bid to avert a military confrontation in Yemen's port city of Hudaida that could worsen the country's already dire humanitarian crisis, Antonio Guterres, the world governing body's chief, had said.

The UN Security Council met behind closed doors on Monday at the request of the United Kingdom to be briefed on the situation after heavy clashes were reported in Yemen's third largest city on Friday and Saturday.

"We are, at the present moment in intense consultation … I hope that it will be possible to avoid a battle for Hudaida," Guterres said.

The UN chief added that Special Envoy to Yemen Martin Griffiths was shuttling between Riyadh and Abu Dhabi.

Griffiths is working on a peace plan that involves the Houthi rebels giving up their ballistic missile arsenal in return for an end to Saudi-led air strikes and a political roadmap, ultimately paving the way for a transition out of the crisis.

Hudaida lies some 230km away from the capital, Sanaa, which is controlled by the Houthis.

In recent days, pro-government forces backed by a Saudi-led coalition have closed in on the port city, driving the rebels from dozens of nearby towns and villages amid growing fears that an all-out assault is imminent.

Aid agencies say it is the port city's location on the Red Sea that makes it such a strategic prize.

If the Saudi-led coalition captures Hudaida, it would effectively be in control of Yemen's most vital lifeline and the main gateway for imports of relief supplies and commercial goods - since the port city is the point of entry for some 70 percent of Yemen's imports.

Riyadh and Abu Dhabi, however, maintain that the port is being used to smuggle weapons.

Mark Lowcock, the head of the UN's office for the Coordination of Humanitarian Affairs, said the consequences of the suspension of operations at the port of Hudaida would be "catastrophic".

"While the UN and other humanitarian agencies are reconfiguring their presence it's also our planned intention though to stay and deliver. We have dozens of UN staff still in Hudaida," Lowcock said.

'Catastrophic impact'

Houthi rebels overran much of the country, including Hudaida, in 2014. The following year, Saudi Arabia, together with several other Arab nations, launched a military campaign in support of Yemen's internationally recognised government aiming to roll back advances made by Houthi rebels after they overran much of the country in 2014.

Most countries have since withdrawn their forces from the coalition, with only Saudi Arabia and the United Arab Emirates conducting attacks in Yemen. The United States is also providing support to the coalition, including air-to-air refuelling and intelligence sharing.

US Secretary of State Mike Pompeo on Monday expressed concern at the situation in Hudaida and called on all parties to "honour their commitments to working with the UN."

"I have spoken with Emirati leaders and made clear our desire to address their security concerns while preserving the free flow of humanitarian aid and life-saving commercial imports," Pompeo said.

On Friday, Lise Grande, the UN Humanitarian Coordinator for Yemen, said that humanitarian agencies "fear, in a prolonged worst case, that as many as 250,000 people may lose everything - even their lives".

More than 10,000 Yemenis have been killed since the war began, tens of thousands have been wounded, and another two million people have been displaced.

An alarming 22.2 million people in Yemen - the Arab world's poorest country - need some kind of humanitarian or protection assistance, according to the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).